This invention relates to the field of self-diagnostic circuits for use in alarm systems and the like, whereby alarm switch means exhibiting erratic switching behavior can be easily identified. More particularly, the invention relates to self-diagnostic circuits for use in alarm systems and the like, or the kind having a plurality of alarm detection switches connected in a loop, means associated with each of the switches for indicating locally the detection of an alarm condition and central control means for reversing voltage polarity in the loop wherever an alarm condition is detected by one of the switches. 2. Prior Art
The self-diagnostic circuit according to this invention is particularly adapted for use in alarm systems and the like, of the kind having a plurality of alarm detection switches connected in a loop, means associated with each of the switches for indicating locally the detection of an alarm condition and central control means for reversing voltage polarity in the loop wherever an alarm condition is detected by one of the switches. Such an alarm system is disclosed in U.S. Patent No. 4,625,198. The alarm system disclosed therein comprises: a plurality of normally-closed electrical contacts wired in series with one another and further connected in series, when in an operating mode, with an alarm condition enunciator. Each of the contacts is mountable at a location to be monitored and is adapted to be opened by occurrence of an alarm condition, the enunciator being operative upon an open-circuit condition at any of said plurality of contacts. The enunciator has electrical polarizing means defining a forward current direction for the series wired contacts in the operating mode. A plurality of individual indicator means are wired one each in parallel with each of said electrical contacts, the individual indicator means having an operational bias means defining an electrical polarity opposite the electrical polarity of the enunciator, whereby a forward current flowing through the series wired contacts will not activate any of the indicator means. Switch means are operable in a test mode to disconnect the enunciator and apply to the series wired contacts a test voltage at a polarity opposite the electrical polarity of the enunciator and cause a reverse current to flow through the series wired contacts, whereby each of a plurality of electrical contacts then in said open-circuit condition is identified by activation of the individual indicator means wired in parallel therewith. The referenced patent also discloses a diagnostic apparatus for use with an existing alarm network of the type described above, the diagnostic apparatus comprising: at least one indicator means connected in parallel with one of the electrical contacts, the indicator means having an operational bias means defining a polarity opposite to the polarity defined by the forward direction of the direct current through the enunciator and contacts; and, a switch operable in a test mode to disconnect the enunciator from the electrical contacts and apply to the series wired contacts an opposite-polarity test voltage to said electrical contacts and cause a reverse current to flow through the series wired contacts, whereupon the at least one indicator means is operable to show an open circuit condition in the electrical contact associated with the at least one indicator means. The teachings of U.S. Patent No. 4,625,198 are entirely incorporated herein by reference.
The alarm system diagnostic apparatus described in the referenced patent provides a number of advantages, including means for easily determining the status of individual contacts in an alarm network and including the ability to identify whether a fault in operation of the alarm system is due to problems with one or more alarm switches or with the wiring connecting the various alarm switches, thereby significantly reducing the time and effort needed to diagnose and repair alarm systems. However, the diagnostic apparatus was incapable of identifying an intermittent but recurring switch fault. Such switch faults might result from broken switch contacts, accidental interruption of a light beam detector or the like, spurious light or sound sources or other randomly occurring events. The diagnostic apparatus is also incapable of identifying the location of an alarm condition after the alarm condition had ceased, and an automatic switch contact had returned to its normally closed position. This might occur if a burglar opened, and then closed a protected window, being scared away by the alarm.
This invention provides a self-diagnostic circuit for use in polarity reversing alarm systems such as those disclosed in the referenced patent, having alarm indicating means which remain active or operable even after an alarm condition ceases, whether or not the alarm condition was a bona fide alarm condition or resulted from spurious operation as described above.
The self-diagnostic circuit according to this invention will not only provide an alarm indicating means which is compatible with the earlier-patented alarm system diagnostic apparatus, but which can be used with so-called "two wire" or "four wire" alarm systems. Two wire alarm systems are those in which only the alarm loop is available for connecting and powering alarm detection switches and the like. The term two wire refers to the fact that each alarm switch or alarm switch circuit is connected to only two wires. In a four wire system, a separate or auxiliary power supply is available to power more sophisticated alarm detection devices, such as infrared detectors, motion sensors and the like. The self-diagnostic circuit according to this invention can be easily adapted for use with both kinds of alarm systems.
Various kinds of alarm systems, some of which include self-latching circuitry, are disclosed in the following United States patents: 3,171,116; 3,314,081; 3,530,455; 3,611,362; 3,766,537; 3,848,242; 3,924,256; 3,978,466; 4,065,762; 4,243,973; 4,303,909; 4,364,030; 4,507,654; and, 4,559,527. It is believed that none of the foregoing patent references discloses a self-diagnostic circuit which includes self-latching circuitry, and which is adapted for use in an alarm system of the kind characterized by central control means for reversing voltage polarity in a loop of series-connected alarm detection switches, each of the switches including means for indicating locally the detection of an alarm condition.